A light emitting diode (LED) often can provide light in a more efficient manner than an incandescent light source and/or a fluorescent light source. The relatively high power efficiency associated with LEDs has created an interest in using LEDs to displace conventional light sources in a variety of lighting applications. For example, in some instances LEDs are being used as traffic lights and to illuminate cell phone keypads and displays.
Typically, an LED is formed of multiple layers, with at least some of the layers being formed of different materials. In general, the materials and thicknesses selected for the layers determine the wavelength(s) of light emitted by the LED. In addition, the chemical composition of the layers can be selected to try to isolate injected electrical charge carriers into regions (commonly referred to as quantum wells) for relatively efficient conversion to optical power. Generally, the layers on one side of the junction where a quantum well is grown are doped with donor atoms that result in high electron concentration (such layers are commonly referred to as n-type layers), and the layers on the opposite side are doped with acceptor atoms that result in a relatively high hole concentration (such layers are commonly referred to as p-type layers).
A common approach to preparing an LED is as follows. The layers of material are prepared in the form of a wafer. Typically, the layers are formed using an epitaxial deposition technique, such as metal-organic chemical vapor deposition (MOCVD), with the initially deposited layer being formed on a growth substrate. The layers are then exposed to various etching and metallization techniques to form contacts for electrical current injection, and the wafer is subsequently sectioned into individual LED chips. Then the LED chips are packaged. Currently employed packaging methods utilize wire bonding, soldering, metal bumps, as well as ultrasonic thermal bonding using silver, tin, and gold bumps, for example.
FIG. 1 shows an exemplary flip-chip packaging 100 of a light emitting device (LED) die employed in the prior art. The light-emitting structure (multiple-quantum well active region 103 sandwiched between p-doped layer 102 and n-doped layer 104) is grown over sapphire substrate 101. Generated in active region 103, light ray 1 undergoes no total internal reflection and escapes through sapphire substrate 101. Generated in active region 103, light ray 2 is totally internally reflected form n-contact layer 105. Reflected ray 2 can escape through the substrate, and eventually about 30% of all generated light can be extracted. N-contact 105 and p-contact 106 are connected to metal submount 108 by solder 107 making an electrical connection between contact 105, 106 and conductive pattern/circuitry 112 of submount 108. Fabricating such packaging structures is a complicated and costly process. More importantly, however, these prior art packaging schemes are not reliable because solder in general and lead free solders in particular are prone to premature failure due to lack of pliability, as well as due to residual stress and deformation during thermal processing.